The present invention relates to a method of evaluating a position error of a movable body such as a gauge head or a tool, and to a method of improving position accuracy by use of the evaluation method, which are used in a three-dimensional coordinate measuring device for moving the gauge head in three axial directions orthogonal to each other or in a moving device such as a machine tool for moving the tool in biaxial or three axial directions orthogonal to each other.
With the advancement of automated and high-accuracy machinery processing, the three-dimensional coordinate measuring device is obliged to have a function of evaluating dimensional accuracy and form accuracy, which is indispensable in a production line and a production system. Meanwhile, increasing of measurement accuracy more than the present level by the three-dimensional coordinate measuring device as hardware incurs a result of raising manufacturing expenses besides the accompanying difficulty in a manufacturing technology. Thus, in recent years, improvement in the measurement accuracy which is a basic performance as a device has been attempted by grasping precision of the device in shipment thereof and by correcting movement of the gauge head.
However, the conventional correction of the movement of the gauge head is the one in which cumulative errors determined when the gauge head is sent for a certain interval are obtained, and then the errors are allocated in proportion to the interval. Thus, the conventional correction is not intended to perform the correction by grasping movement errors of the gauge head within the interval. The above-described is symbolized by the fact described as follows. That is, in the present situation, the evaluation method itself of the movement errors is in the stage of evaluating and comparing the errors of the measuring device in such a manner that a ball plate as shown in FIG. 12, which is an accuracy standard and is taken around by the world""s leading organizations, is measured as shown in FIG. 13 by three-dimensional coordinate measuring devices possessed by the world""s leading organizations for searching for a standard method.
Incidentally, the ball plate is very expensive and has a considerable weight. Thus, handling thereof has not been easy. In addition, in tests by taking around the ball plate, the result thereof has not reached the point of obtaining a result as systematic as error characteristics of the three-dimensional coordinate measuring device can be stipulated. Note that FIGS. 14(a) and 14(b) are examples of displaying results of error measurement by the ball plate. FIG. 14(a) shows directions and sizes of errors by use of bars; FIG. 14(b) shows errors within a measured plane by deformation of meshes. In order to obtain position errors in three axial directions of the gauge head by locating the ball plate at a specified position in a space, it is necessary to repeat adjustment of highly accurate positioning of the ball plate of which handling is hardly easy. The above can be hardly achieved in reality, and it is extremely difficult to obtain an error space by the foregoing.
Although space errors are not obtained, as a standard error calibration method available for practical use, enumerated are: a method by use of a step gauge using standard blocks as shown in FIG. 15; a method by use of a normal block gauge; a method by use of a test bar as shown in FIG. 16; a method by use of an autocollimator as shown in FIG. 17; and a method by use of a laser measuring device as shown in FIG. 18. However, in conventional methods such as the above-described method by use of the test bar, method by use of the autocollimator, laser measuring device, ball plate, and step gauge using the standard blocks, respectively, or a reverse method as shown in FIGS. 19(a) and 19(b), there are problems that adjustment thereof takes long time, automatic evaluation is hard to perform, and the accuracy of the measuring devices is hard to maintain.
Meanwhile, considering the case of machine tool, besides the autocollimator and a straight ruler, the laser measuring device is used for evaluating movement accuracy of a tip of a tool. However, in reality, it is hard to obtain the error space by using the above because of the following reasons and the like. Specifically, disposition and adjustment of the devices require time, the devices are not for use in evaluation of tool movement even though they are suitable for evaluating accuracy of work pieces, and the devices require too much work and time in identifying errors in three axial directions of a predetermined position in a space.
The various methods which have been heretofore used are the ones in which due consideration is given in terms of evaluating accuracy. However, from the view point of operability, productivity, price or the like concerning measurement, the above devices are not necessarily proper to be used for various purposes or to be standard devices. Therefore, it was actually hard to achieve improvement in accuracy of the device by correcting movement of the gauge head in such a manner that the error space is evaluated by the conventional method and set as a fundamental error characteristic to be an object of the correction, and a function of hardware is secured in a certain level.
Incidentally, the inventors of the present application have proposed a method of measuring a straightness error by use of a sequential two-point method in the article xe2x80x9cTrend of Straightness Measuring Method and Development of Sequential Two-Point Methodxe2x80x9d which was previously presented in the pages 25 to 34 of xe2x80x9cProduction Researchxe2x80x9d Vol. 34, No. 6, published in June of 1982 by Institute of industrial Science, University of Tokyo. The sequential two-point method is the one for obtaining a straightness error of movement of a tool stage and a straightness error of a surface of an object to be measured simultaneously and independently of each other. Specifically, the sequential two-point method is carried out in the following manner: two displacement sensors disposed with a space therebetween on the tool stage are moved in a direction of the space at a pitch equal to the space, and simultaneously, a displaced quantity of each displacement sensor with respect to the surface of the object to be measured is measured, and thus the above straightness errors are obtained from data rows of the displaced quantities of the two displacement sensors. The inventors of the present application have achieved the points that, by application of the sequential two-point method to evaluation of errors as described above, the adjustment takes less time, the automatic evaluation can be performed easily, and the accuracy of the measuring device is easily maintained, compared to the conventional methods such as the method by use of the test bar, the methods by use of the autocollimator, laser measuring device, ball plate, and step gauge using the standard blocks or the reverse method as shown in FIGS. 19(a) and 19(b).
The present invention is intended to provide an error measuring method which has solved the problem of the conventional method advantageously in consideration for characteristics of the above-described sequential two-point method. A position error evaluating method of a moving device according to the present invention is characterized by including the following steps. Specifically, according to a method specified in claim 1, in a moving device which moves a movable body in two axial directions or in three axial directions orthogonal to each other, obtaining by the sequential two-point method a straightness error curve indicating a state of change in a position error of the movable body along a uniaxial direction out of predetermined two axial directions is repeated for the other uniaxial direction out of the predetermined two axial directions, the position error being related to a direction orthogonal to the predetermined two axial directions out of the biaxial or three axial directions. Subsequently, straightness error curves indicating a state of change in a position error of the movable body along the other uniaxial direction are obtained based on coordinate positions of both ends of a group of already obtained straightness error curves, the position error being related to the direction orthogonal to the predetermined two axial directions. The straightness error curves at the coordinate positions of the both ends are set as boundary straightness error curves. Thereafter, based on the boundary straightness error curves, alignment of the group of straightness error curves is corrected, thereby obtaining an error surface. Lastly, in accordance with the error surface, a two-dimensional position error of the movable body on a planar surface including the predetermined two axes is evaluated, the two-dimensional position error being related to a direction orthogonal to the planar surface.
According to the position error evaluating method of the moving device of the present invention, for error evaluation of a three-dimensional coordinate measuring device and a device with a similar structure thereto of measuring a semiconductor substrate, a glass substrate for a liquid crystal display device and the like, straightness error curves and a planar error surface of movement of a gauge head can be measured. In addition, besides the above, the sequential two-point method has a characteristic of being capable of simultaneously measuring an error shape of a substrate to be measured and the like. According to the method of the present invention, with regards to the moving device other than the measuring device, such as a machine tool, the straightness error curve and planar error surface can be measured for the error evaluation of the movement of the movable body such as a tool stage.
Note that, according to the position error evaluating method of the moving device of the present invention, as specified in claim 2, an error space may be obtained in such a manner that obtaining the error surface by the method specified in claim 1 with respect to the uniaxial direction orthogonal to the planar surface including the predetermined two axes is repeated within predetermined coordinate ranges in each of the three axial directions. Accordingly, a three-dimensional position error of the movable body in a space within the predetermined coordinate range may be evaluated according to the error space.
According to the position error evaluating method of the moving device of the present invention, while taking as the basics a straight movement error curve of the gauge head as the movable body, the error space is obtained by the sequential two-point method as described above. Thus, a performance related to a measurement error can be evaluated more in detail. Identification of the error space with the straightness error curve obtained by the sequential two-point method as the basics according to the method of the present invention is excellent in the following points. Specifically, the identification of the error space is comprehensive as the space, adjustment of instruments and the like for implementing measurement takes less time, and systematic errors corresponding to coordinate axes can be obtained.
Moreover, according to the position error evaluating method of the moving device of the present invention, as specified in claim 3, at an intermediate point of the points where the position error has been obtained by the sequential two-point method, a position error may be obtained by one-dimensional or multidimensional interpolation based on characteristics of the position error.
In the above-described manner, at the intermediate point of the points where the position error has been obtained by the sequential two-point method, the position error is obtained by the interpolation. Thus, even if the points where the position error is obtained by the sequential two-point method are spaced to some extent, the position error can be obtained at the point between those points. Therefore, the speed of the measurement by the sequential two-point method can be increased, and the straightness error curve and the error surface can be obtained in a shorter period of time.
A method of improving three-dimensional position accuracy of a moving device of the present invention according to claim 4 is characterized by including the steps of: maintaining data in a control device for controlling an operation of the moving device, the data indicating the error space obtained by the method specified in claim 2 or 3; and, by use of a relational expression of correction for compensating for an error in the above error space data correcting a position of the movable body moved by the moving device.
According to the method of improving three-dimensional position accuracy of a moving device of the present invention, the data indicating the error space obtained by the sequential two-point method as described above is maintained in the control device, and a motion, based on the data maintained in the control device, of correcting the error by use of the relational expression of correction for compensating for the error in the error space data is given to the moving device by, for example, a CNC (computer numeric control) function of the control device. Accordingly, improvement in measurement accuracy and movement accuracy of a tool, which are actual functions of the device, can be achieved. As a result, in the state of reaching the stage where price rise is inevitable in the case of improving accuracy by the hardware configuration of the device, improvement in the device performance can be expected while restraining the price rise.
For a semiconductor substrate, a glass substrate for a large-sized image display device, and the like, accuracy evaluation in a planar shape of nm order is required. For the above accuracy evaluation, considered is the employment of a three-dimensional coordinate measuring device of a normal rectangular coordinate system using a machine platen as a table or of a three-dimensional coordinate measuring device of a polar coordinate system which rotates a table. In the above devices, maximum accuracy is upgraded by hardware modification, thereby aiming for achievement of required measurement accuracy and resolution. Therefore, by introducing the sequential two-point measuring method to the above devices and by applying the method of the present invention thereto, a function of further higher accuracy can be offered to the measuring device derived from the idea of the conventional method. Accordingly, contribution can be made to improvement of processing accuracy in a production technology.
Note that, when the position error evaluating method of the present invention is applied to the three-dimensional coordinate measuring device of the polar coordinate system, instead of three axes orthogonal to each other, two axes orthogonal to each other and a rotation angle around one axis out of the two axes may be used. In the above case, position error data measured by moving a displacement sensor along an orthogonal coordinate system may be converted to the polar coordinate system. Meanwhile, the position error data may be directly obtained by moving the displacement sensor radially and circumferentially along the polar coordinate system.
Furthermore, in the method of improving three-dimensional position accuracy of the moving device of the present invention, as specified in claim 5, a position of the movable body moved by the moving device may be corrected upon using the moving device by previously obtaining the error space data according to a change of environment surrounding the moving device and by using a relational expression of correction for compensating for an error in the error space data corresponding to environment surrounding the moving device in use.
A matrix structure of the error space is assumed to be changed in some cases by surrounding environment, use conditions and the like. In regard to the above, because the error space can be automatically and easily obtained by the sequential two-point method compared to the conventional method, easy correction can be executed by imparting a matrix structuring an error space to the changes of the above conditions.
Specifically, as specified in claim 5, the error space data is previously obtained and stored, the error space data being relative to the surrounding environment, typically the ambient temperature of the device, temperature rise in a representative point of the structure due to intensive and repeated use, and the like. Thus, by monitoring the environment in a real environment, correction for each error space can be performed. Accordingly, against the change in the surrounding environment of the measuring device or the machine tool, measurement accuracy and movement accuracy of a tool can be maintained.